1. Field of the Invention
The invention pertains generally to optical fiber communication and sensing systems.
2. Art Background
Optical fibers are now widely employed in optical communication systems, and have been proposed for use in a variety of sensing systems. For example, and as schematically depicted in FIG. 1, an optical fiber communication system 10 typically includes an optical source 20, e.g., a semiconductor laser, which communicates with an optical detector 40 through an optical fiber 30 (typically enclosed within a cable). That is, the optical fiber 30 serves to transmit at least a portion of the light, e.g., information-carrying light pulses, emitted by the optical source to the detector. By contrast, and as schematically depicted in FIG. 2, an optical fiber sensing system 50 typically includes an optical source 20 which communicates with a sensor (a transducer device) 60 through an optical fiber 30. In addition, the system 50 includes an optical detector 40 which communicates with the sensor 60 through, for example, the optical fiber 30 (as shown in FIG. 2) or through a second optical fiber. In operation, the optical fiber 30 transmits at least a portion of the light emitted by the optical source 20 to the sensor 60. At least a portion of the transmitted light is either reflected by the sensor 60 back into the optical fiber 30, or is reflected or transmitted by the sensor 60 into the second optical fiber, and thus transmitted to the optical detector 40. If an appropriate external stimulus impinges upon the sensor 60, then the sensor (a transducer device) typically alters the intensity and/or phase of the light transmitted to the detector 40. Significantly, the sensor 60 often includes a portion of one or both of the optical fibers. (Regarding these sensing systems see, e.g., Technical Digest 3rd International Conf. On Optical Fiber Sensors, San Diego, Ca. 1985.)
Two factors often play a significant role in the operation and/or design of the optical fiber systems described above. One of these is the optical loss per unit length (hereafter termed optical loss or just loss, typically measured in decibels per kilometer (dB/km)) suffered by an optical signal, e.g., an optical pulse, within the optical fiber. Typically, at present, this factor determines the distance between signal amplifiers (repeaters) along the length of the optical fiber. As is known, the optical loss is due to both intrinsic and extrinsic losses. The former is associated with the intrinsic properties of the material employed in the optical fiber. The latter denotes all other losses.
Included among the extrinsic losses are microdeformation and macrobending losses. Microdeformation losses denote the scattering losses produced by random microbends of the fiber axis (bends, i.e., deviations from perfect straightness, typically having magnitudes less than about 1 .mu.m), as well as random fluctuations (typically smaller than about 0.1 .mu.m) in the fiber core diameter. Macrobending losses are radiative losses produced by macroscopic bends (bends having radii or curvature typically larger than about 0.3 cm) in the fiber. (Regarding microdeformation losses see, e.g., D. Marcuse, Appl. Optics, 23 1082 (1984). Regarding macrobending losses see, e.g., L. G. Cohen, et al, IEEE J. of Quantum Electronics, Vol. QE-18, No. 10, page 1467, 1982.)
At present, silica (SiO.sub.2) glass is the material which is employed in almost all optical fibers. The intrinsic loss of silica is wavelength dependent (as is the case with most other materials) and exhibits a minimum of about 0.16 dB/km at a wavelength of about 1.55 .mu.m. Thus, to maximize repeater spacing, i.e., to achieve a repeater spacing as large as about 200 kilometers (km), silica based optical fiber systems are often operated at the minimum intrinsic loss wavelength, i.e., 1.55 .mu.m. Significantly, the extrinsic losses associated with present-day silica fibers are much smaller than the intrinsic loss. For example, the microbending loss associated with silica glass fiber (also wavelength dependent) is estimated (by the present inventors) to be (for a single mode silica fiber) only about 5 percent of the intrinsic loss at 1.55 .mu.m. As a consequence, microbending losses have been ignored, or have had little impact, on the design of silica based optical fiber systems.
The second factor which has significantly impacted the design of the optical fiber systems described above, is the dispersion (typically measured in units of picoseconds/kilometers-nanometer (ps/km-nm)) suffered by optical signals within an optical fiber. Dispersion limits the information flow rate, e.g., bit rate, through an optical fiber. As is known, total dispersion includes modal dispersion, material dispersion, and waveguide dispersion. Modal dispersion denotes the dispersion due to the different propagation speeds of the different modes guided by the core of an optical fiber. Material dispersion denotes the dispersion due to the wavelength dependence of the refractive index of the optical fiber material. Waveguide dispersion (which exists even in the absence of material or modal dispersion) denotes the dispersion arising from the different spatial electromagnetic power distributions assumed by different wavelength signals within the optical fiber. For example, one wavelength signal may have a spatial power distribution in which power transmission is largely confined to the cladding, while another wavelength signal may have a spatial power distribution in which power transmission is largely confined to the center of the core. The two wavelength signals will necessarily "see"]different average refractive indices, and thus propagate at different average speeds. The spatial extent of electromagnetic power, and thus the waveguide dispersion, varies with the transmission wavelength, as well as with the physical and material characteristics of the fiber, e.g., the core radius, the relative refractive index difference between core and cladding, and the absolute refractive index of the fiber material. Significantly, for many fiber materials and specific fiber parameters, there are wavelength regions where the waveguide dispersion is of opposite sign (i.e., acts in opposition) to that of the material dispersion.
Modal dispersion in silica fibers has been eliminated by fabricating single mode silica fibers, i.e., fibers in which the core guides only a single mode. If, for example, the core and cladding of a fiber have different but uniform refractive indices (with the refractive index of the core being higher than that of the cladding to achieve waveguiding) then, as is known, single mode operation is achieved provided ##EQU1## Here, a denotes the core radius, n.sub.cl denotes the refractive index of the fiber cladding material, .DELTA.=(n.sub.c -n.sub.cl)/n.sub.c, where n.sub.c is the refractive index of the core, and .lambda. denotes the transmission wavelength. (Regarding the requirement of Equation (1) see, e.g., D. Marcuse et al, in Optical Fiber Telecommunications edited by S. E. Miller and A. G. Chynoweth (Academic Press, New York, 1979), Chapter 3. )
Although silica fibers exhibit zero material dispersion at a wavelength of 1.27 .mu.m rather than at 1.55 .mu.m (the minimum intrinsic loss wavelength), single mode silica fibers have been developed which exhibit very low (typically less than about 1 ps/km-nm) total dispersion at 1.55 .mu.m. This has been achieved by fabricating dispersion-shifted silica fibers, i.e., silica fibers in which waveguide dispersion is used to counterbalance (negate) material dispersion at a desired wavelength, e.g., the minimum intrinsic loss wavelength. (Regarding dispersion shifting see, e.g., L. G. Cohen et al, Electr. Lett., 15,334 (1979).)
Recently, significant interest has been generated in developing single mode, dispersion shifted optical fibers based on long wavelength materials, i.e., materials transparent to light at wavelengths ranging from about 2 .mu.m to about 11 .mu.m. (As with silica, the zero material dispersion wavelength of these long wavelength materials differs from the minimum intrinsic loss wavelength.) Such materials include glasses such as zirconium fluoride based glasses and zinc chloride based glasses. These long wavelength materials are believed to exhibit minimum intrinsic losses (for wavelengths ranging from about 2 .mu.m to about 11 .mu.m) as low as, or even lower than, 0.01 dB/km. (By contrast, silica exhibits a minimum intrinsic loss of 0.16 dB/km at 1.55 .mu.m.) Consequently, these long wavelength materials offer the possibility of producing optical communication and sensing systems having more widely spaced, and thus fewer, repeaters, i.e., repeaters spaced more than about 200 km, even more than about 400 km, and even more than about 1000 km, apart from one another.
In contrast to silica based optical fiber systems, long wavelength materials have intrinsic losses which are so low that extrinsic losses, such as microdeformation losses, are often as large as, or larger than, the intrinsic losses. As a consequence, to attain the potential benefits inherent in long wavelength materials, significant efforts have been devoted to developing optical fiber designs which reduce microdeformation losses.
A generally accepted theory for predicting microbending losses (for any material) has been developed by Klaus Petermann. (See Klaus Petermann, "Theory of Microbending Loss in Monomode Fibres with Arbitrary Refractive Index Profile," AEU Arch. Elektron Uebertragungstech. Electron Commun., 30, 337 (1976).) On the basis of this theory, and assuming constant V number, it can readily be shown that the microbending loss, .alpha..sub.M, for a single mode fiber is proportional to (denoted by the symbol .alpha.) ##EQU2##
Equation (2) indicates that at a fixed wavelength, .lambda., and for a particular material, i.e., for a fixed refractive index, n the microbending loss, .alpha..sub.M, increases as .DELTA. is decreased. Equation (2) also indicates that for a fixed n and .DELTA., .alpha..sub.M increases as transmission wavelength, .lambda., is increased. Thus, just to keep .alpha..sub.M constant as .lambda. is increased, .DELTA. must increase with .lambda., i.e., EQU .DELTA..alpha..lambda..sup.2/3.
Consequently, on the basis of the Petermann theory, it has been believed that the only way to reduce microbending losses for a given material operating at a given wavelength (longer than about 2 .mu.m) is to employ relatively large values of .DELTA.. Moreover, it has been believed that the .DELTA. values must increase (beyond the initially large values) as transmission wavelengths are increased, i.e., as longer wavelength materials are employed. Significantly, according to the Petermann theory, it is the very long wavelength materials (which are believed to have the lowest intrinsic losses) which must have the very largest .DELTA. values to reduce microbending losses.
The values .DELTA. values (expressed as a percent) employed in silica fibers are typically a few tenths of a percent (%), e.g., 0.3%. In the case of single mode, dispersion shifted (to 1.55 .mu.m) silica fibers, .DELTA. values of 0.50% or higher have been necessary. By contrast, the Petermann theory typically imposes the requirement that the .DELTA. values (at wavelengths greater than about 2 .mu.m) be as much as, or even more than, 10 times the values used in silica fibers, e.g., 3% or 5%. But, at present, such large .DELTA. values are extremely difficult, and in some cases impossible, to achieve in long wavelength materials. Moreover, even if such large .DELTA. values were achievable, it is known that they would result in substantial crystallization at the core-cladding interface during fiber manufacture, which significantly increases loss. Consequently, the Petermann theory has led to an impasse, i.e., according to the Petermann theory, microbending losses can only be reduced by using large .DELTA. values which are either impossible to achieve or, if achievable, lead to crystallization and thus higher losses.
Thus, those engaged in the development of optical fiber systems based on long wavelength materials have sought, thus far without success, practical methods for reducing microbending losses, while also achieveing low dispersion.